Multi-PeV Signals from a New Astrophysical Neutrino Flux Beyond the Glashow Resonance

The IceCube neutrino discovery was punctuated by three showers with $E_ u$ ~ 1-2 PeV. Interest is intense in possible fluxes at higher energies, though a marked deficit of $E_ u$ ~ 6 PeV Glashow resonance events implies a spectrum that is soft and/or cutoff below ~few PeV. However, IceCube recently reported a through-going track event depositing 2.6 $pm$ 0.3 PeV. A muon depositing so much energy can imply $E_{ u_mu} gtrsim$ 10 PeV. We show that extending the soft $E_ u^{-2.6}$ spectral fit from TeV-PeV data is unlikely to yield such an event. Alternatively, a tau can deposit this much energy, though requiring $E_{ u_tau}$ ~10x higher. We find that either scenario hints at a new flux, with the hierarchy of $ u_mu$ and $ u_tau$ energies suggesting a window into astrophysical neutrinos at $E_ u$ ~ 100 PeV if a tau. We address implications, including for ultrahigh-energy cosmic-ray and neutrino origins.